There is currently great interest in the transmission of information without electrical contacts between a system supplied by a power source, master and slave(s) that receive the operational power by way of signals generated by the primary system. One example of these applications is the RFID and near-field induction communication systems. All these systems are based on the master generating an electromagnetic field, by means of an antenna system, captured by antennas situated on the slave. The voltage generated in these antennas is used to power the circuit and transmit data from the master to the slave. This transmission can be carried out by means of modulating the amplitude, frequency, phase, etc. The modes of transmission from the slave to the master are very varied, including:    a) storing energy in a capacitor and using it to generate an electromagnetic field to transmit the data by modulating the amplitude, frequency, phase . . . this method involves the master transmitting a field, shutting down the transmission and upon completing this, the slave generates its response generally at the same working frequency of the master.    b) modulating the energy dissipated by the antenna of the slave, which is translated into changes in the quality factor of the transmitter circuit of the master.    c) using two antennas in the slave: one transmitting and another receiving. This allows master-slave transmission at one frequency and slave-master transmission at a different frequency.
In general, all these transmission methods are based on resonant systems where the antenna, which is generally a loop antenna, is coupled to a capacitor to syntonize a determined frequency. In some cases, in order to reduce the size of the antenna, a winding with a ferritic core is used, maintaining the antenna working as a linear device.
The closest prior art is the system described in the Spanish patent with publication number ES 2251315 B2. It is based on detecting a magnetic material magnetized by an external exciter field generated by the master. The magnetic material is situated inside a short-circuitable cable such that the magnetization of the material is modified upon opening and closing the circuit. These magnetization changes at the same frequency as the exciter field are detected by a dual-phase demodulation system. The master and the slave have the same transmission and reception frequency, therefore a system that is just as sensitive in detection as the dual-phase demodulation is required since the exciter field is much greater than that produced by the magnetic core, this also requires excellent signal compensation.